[adorno]

There is no reason whatsoever for the phone part to be detached.

Perhaps not, but, it would be a lot more convenient to be using the "detached" phone component, while also using the phone's provider of service (the small form factor PC) for many other things at the same time. When a device is brought up to one's ears, the form factor becomes, mostly, just a phone. A PC that can serve as a PC and a tablet and that can be used as a "super-computer", should not be constrained to a simple phone, just because it carries the phone component internally.

[MrEdd]

Carrying more devices is not more convenient. Especially for people who either;

1. Don’t make many calls.
2. Make enough calls to use a headset anyway.

I am in category 1.

I carry my phone in the front pocket of a fishing shirt, opposite the Kindle in the other pocket. I use a headset so I can talk while working. The few calls I get are from management.